PPU palettes

The palette for the background runs from VRAM $3F00 to $3F0F; the palette for the sprites runs from $3F10 to $3F1F. Each color takes up one byte.

$3F00	Universal background color
$3F01-$3F03	Background palette 0
$3F05-$3F07	Background palette 1
$3F09-$3F0B	Background palette 2
$3F0D-$3F0F	Background palette 3
$3F11-$3F13	Sprite palette 0
$3F15-$3F17	Sprite palette 1
$3F19-$3F1B	Sprite palette 2
$3F1D-$3F1F	Sprite palette 3

Addresses $3F04/$3F08/$3F0C can contain unique data, though these values are not used by the PPU when rendering. Addresses $3F10/$3F14/$3F18/$3F1C are mirrors of $3F00/$3F04/$3F08/$3F0C.

Another way of looking at it:

43210
|||||
|||++- Pixel value from tile data
|++--- Palette number from attribute table or OAM
+----- Background/Sprite select

Like most early color game consoles, the NES palette is based on Hue/Saturation/Value

76543210
||||||||
||||++++- Hue
||++----- Value
++------- Unimplemented, reads back as 0

Hue $0 is light gray, $1-$C are blue to red to green to cyan, $D is dark gray, and $E-$F are black. The canonical code for "black" is $0F. It works this way because of the way colors are represented in an NTSC or PAL signal, with the phase of a color subcarrier controlling the hue. For details, see NTSC video.

Note that most VS Unisystem arcade PPUs have completely different palettes, and Playchoice-10 PPUs render hue $D as black.

If rendering is disabled in PPUMASK ($2001), and the current VRAM address points in $3F00-$3FFF, this color will be shown on screen instead of the backdrop color. A loop that fills the palette will cause each color in turn to be shown on the screen. So to avoid horizontal rainbow bar glitches while loading the palette, wait for a real vertical blank first using an NMI technique.